The increasing compact nature of portable digital media players has driven the development of hard disk drives of decreasing sizes. However, despite the small sizes of hard disk drives, they still function the same way as large disk drives. All disk drives employ a servo system which controls the radial position of an actuator arm relative to a rotating recording disk. The actuator arm supports a read/write head or transducer in cantilevered fashion above the disk. Ideally, the read/write head is maintained over the center of a selected track in the disk surface. The read/write head senses position information from the track, which is then used to develop a position error signal. The error signal is fed back through a compensator into a drive motor for the actuator arm to move the read/write head in a direction to reduce the error.
Disk drives used in portable digital media players may experience an immense shock from being dropped or severely jolted. For example, when the media player containing a small disk drive (such as a 1 inch disk drive) falls from a desk or user's hand onto a hard floor, the impulse generated at impact can be quite large—up to hundreds or thousands of g's. Such disturbances are in the form of vibrations which could, if sufficient in strength, cause the disk to move relative to the read/write head, resulting in an off track error.
The external shock yields two undesirable outcomes: physical damage and track mis-registration. With respect to the first, the shock pulse may cause significant physical damage to the disk drive. A pulse of sufficient magnitude and orientation can overcome the air gap bearing which separates the slider from the rotating disk, thereby damaging one or more of: the magnetic media film, the disk substrate, or the slider.
With respect to the second, the external disturbance imposes linear and radial accelerations that apply a moment to the actuator arm. This moment may exceed the tracking servo system's ability to maintain the read/write head within the allocated tracking error range required for acceptable data integrity. This problem is exacerbated by the increased track density of smaller disk drives which reduces the acceptable tracking error range. If a shock occurs during the data writing process, the disk drive is in jeopardy of miswriting the data off track, or worse, over previously written data on adjacent tracks.
The present invention concerns disk drives and associated methods for detecting shock-inducing events, such as free fall conditions, and taking protective action to minimize the damage and errors induced by such shock.